Electro-optic modulators are used for high-speed optical communication systems and ultrafast information processing applications. Bulk modulators of discrete electro-optic materials are possible, but typically with high driving voltage and narrow modulation bandwidth. To lower driving voltage and increase modulation bandwidth, materials with large electro-optic coefficients, low dielectric constants and single mode waveguide formation are required.
The ferroelectric oxide, LiNbO3, has been widely utilized in low loss, low drive voltage high-speed modulators because of its large electro-optic coefficients and excellent optical transparency. Low loss optical waveguides can be produced by locally modifying the composition or stoichiometry in the LiNbO3 substrate through diffusion or ion-exchange processes. Other ferroelectric materials with much higher electro-optic coefficients could lead to improved electro-optic modulators with lower drive voltages and smaller dimensions. Of particular interest is the ferroelectric BaTiO3, which has electro-optic coefficients as high as 1300 pm/V in the bulk (two orders higher than that of LiNbO3). Bulk BaTiO3, however, has a very large dielectric constant that limits its application for high modulation bandwidth. Further restricting such use, the metal-diffusion and ion-exchange fabrication methods adapted for LiNbO3 have not been successful with bulk BaTiO3 material. As a result, thin film ferroelectric BaTiO3 waveguide modulators have been developed.
A thin film composite structure of BaTiO3/MgO has been shown to lower the effective microwave index enabling velocity match between the microwave and optical waves needed for high-speed traveling wave operation. Furthermore, BaTiO3 thin films can be integrated with silicon using a thin film buffer layer of MgO. However, fabrication of low-loss optical waveguides with BaTiO3 thin films on a low index substrate has proven a difficult task due to optical scattering losses. The same high refractive index contrast that enables waveguides with tight bends and small dimensions also increases sensitivity to surface scattering. Conventional methods for thin film ridge waveguide formation, such as directly wet-etching or dry-etching into the BaTiO3 thin film, increase the surface and sidewall roughness of the BaTiO3 waveguide. Optical waveguides fabricated by these methods exhibit a propagation loss of 3–4 dB/cm and a high polarization dependent loss on the order of 5 dB/cm. Polarization-dependent loss increases the complexity of a fiber communication system. Accordingly, the development of polarization insensitive operational waveguides is an extremely important component in the advancement of such systems.